Within the scope of large-scale utilization of photovoltaics, which has been developing explosively in recent years, as an alternative method for generating power, solar cells based on crystalline silicon are of outstanding importance as ever. This is caused primarily by their high efficiency, but also by their proven long service life at high yield, as well as by the established and highly productive technology for producing them.
Still, competition with other methods of generating power and the dramatic reduction in politico-economic subsidies offered, forces the producers to constant cost reductions. Since the cost reduction potential is limited on the material side, there is a search for possibilities of efficiency increases in the production process of solar cells as well as subsequently situated steps of the formation of solar modules and solar panels.
In solar cells having a monocrystalline silicon substrate (especially based on CZ silicon), selectively doped emitters having a finger-like pattern have become established, which are especially formed by phosphorus doping.
In the case of the present Applicant, a process flow for producing solar cells was developed having a selective emitter according to the concept of a “patterned” source, as shown in Table 1A below. In this process flow, the wafer is first coated with phosphorus silicate glass (PSG), which is subsequently masked with a wax by inkjet pressure and is removed from the areas between the fingers in a wet-chemical manner. After removal of the wax masking, a second drive-in step takes place in an oxygen atmosphere, by which step the doping is driven deeply into the regions between the fingers and the surface concentration is reduced at the same time. Since at the time of this driving in there is no phosphorus silicate glass in these regions, a relatively pure oxide grows which may be used in an SiO/SiN stack for passivation.
By the combination of a deeply driven in emitter having good oxide passivation, very low emitter saturation currents and a good blue sensitivity are able to be achieved. One disadvantage of this method is the high cost of the inkjet masking. This is based on the material usage (wax), and the necessity for a separate wet step for the removal of the wax.
One alternative method is to apply laser doping according to T. C. Räder, P. Grabitz, S. J. Eisele, J. R. Köhler, and J. H. Werner, “0.4% Absolute Efficiency Gain of Industrial Solar Cells by Laser Doped Selective Emitter”, 34th Photovoltaic Specialists Conf., edited by: IEEE Publishing Service, Piscataway, N.J. (2009), having a one-step diffusion, as shown in Table 1B below. In this instance, using a PSG coating and subsequent diffusion, first a relatively high-resistant flat diffusion is produced. Subsequently, with the aid of a laser, in the finger region the silicon is melted to a depth of a few 100 nm, whereby a greater quantity of phosphorus is able to penetrate into the substrate from the PSG present.
This method is clearly less cost-intensive than the concept, explained above, of the “patterned” source using an inkjet pressure method, since no resist is required for the patterning and there is only one high temperature step. A disadvantage is, however, that because of the one step diffusion, only a qualitatively poorer emitter is able to be produced. This is based on the fact that no deep diffusion is able to be achieved, since the surface has to remain coated with highly doped PSG. Also, the highly doped PSG cannot be used for passivating, so that the passivating has to be performed using qualitatively poorer SiN. One further problem is crystal defects in the laser-doped region, which increase the recombination in the metallized region.
The two abovementioned concepts are briefly clarified in the following tabular compilation of Tables 1A and 1B:
TABLE 1APatterned Source using Inkjet Method:1texture2coating with POCl33printing of finger region4PSG etching (HF)5stripping of inkjet resist6driving in emitter (O2)7oxide etching8PECVD nitride VS9printing10firing
TABLE 1BLaser Doping after Emitter Diffusion:1texture2shallow emitter diffusion (POCl3)3laser doping in the finger region4PSG etching (HF)5PECVD nitride VS6firing